Fastening devices, systems, and methods

ABSTRACT

A process of forming a fastener with improved threading to resist multi-axial forces and off-axis loading scenarios is provided. The process may include placing first and second mill tools adjacent a shaft of the fastener having a proximal end and a distal end, rotating the shaft and the first and second mill tools, and translating the first and second mill tools along at least part of a length of the shaft to form: a first concave undercut surface oriented toward the proximal end, a first convex undercut surface oriented toward the distal end, a second concave undercut surface oriented toward the distal end, and a second convex undercut surface oriented toward the proximal end.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 63/116,092 filed on Nov. 19, 2020, entitled“FASTENING SYSTEMS AND METHODS”, and U.S. Provisional Patent ApplicationSer. No. 63/147,640 filed on Feb. 9, 2021, entitled “FASTENING DEVICES,SYSTEMS, AND METHODS”. The above-referenced documents are herebyincorporated by reference in their entirety.

TECHNICAL FIELD

The present disclosure relates to fastening devices, systems, andmethods. More specifically, the present disclosure relates to fasteningdevices with improved thread designs, fastening systems utilizingfastening devices with improved thread designs, and methods ofmanufacturing fasteners with improved thread designs.

BACKGROUND

Surgical procedures involving fasteners implanted within bone and othertissues can become lose over time due to multi-axial forces and off-axisloading scenarios that may be applied to the fastener during the healingprocess. Traditional fastener thread designs may not provide sufficientfastener fixation to overcome these multi-axial forces and off-axisloading scenarios.

Accordingly, fasteners with improved thread designs for increasing bonefixation and load sharing between a bone/fastener interface experiencingmulti-axial and off-loading conditions, as well as improved methods formanufacturing such fasteners, would be desirable.

SUMMARY

The various fastening devices, systems, and methods of the presentdisclosure have been developed in response to the present state of theart, and in particular, in response to the problems and needs in the artthat have not yet been fully solved by currently available fasteningdevices, systems, and methods. In some embodiments, the fasteningdevices, systems, and methods of the present disclosure may provideimproved bone fixation and load sharing between a bone/fastenerinterface under multi-axial and off-loading conditions.

In some embodiments, a process for forming a fastener may include:placing a first cutting head of a first mill tool at a first positionalong a substantially cylindrical substrate having a proximal end and adistal end, placing a second cutting head of a second mill tool at asecond position along the substantially cylindrical substrate, rotatingthe first cutting head about a first longitudinal axis of the first milltool, rotating the second cutting head about a second longitudinal axisof the second mill tool, rotating the substantially cylindricalsubstrate about a third longitudinal axis of the substantiallycylindrical substrate, and translating the first and second cuttingheads along at least part of a length of the substantially cylindricalsubstrate to form a first concave undercut surface oriented toward theproximal end, a first convex undercut surface oriented toward the distalend, a second concave undercut surface oriented toward the distal end,and a second convex undercut surface oriented toward the proximal end.

In some embodiments of the process, translating the first and secondcutting heads along at least part of the length of the substantiallycylindrical substrate may form a first helical thread and a secondhelical thread. The first helical thread may include the first concaveundercut surface and the first convex undercut surface. The secondhelical thread may include the second concave undercut surface and thesecond convex undercut surface.

In some embodiments, the process may also include placing the firstcutting head adjacent the second cutting head along a side of thesubstantially cylindrical substrate with the first longitudinal axis ofthe first mill tool substantially parallel to the second longitudinalaxis of the second mill tool.

In some embodiments of the process, the first cutting head may includeat least one convex cutting surface and the second cutting head mayinclude at least one concave cutting surface.

In some embodiments of the process, the at least one convex cuttingsurface may include a first facet and a second facet, and the at leastone concave cutting surface may include a third facet and a fourthfacet. The first facet and the second facet may be angled with respectto each other by a first angle that may be greater than 180 degrees toform the at least one convex cutting surface, and the third facet andthe fourth facet may be angled with respect to each other by a secondangle that may be less than 180 degrees to form the at least one concavecutting surface.

In some embodiments, the process may also include placing the firstcutting head on a first side of the substantially cylindrical substrateand placing the second cutting head on a second side of thesubstantially cylindrical substrate. The first cutting head and thesecond cutting head may be separated by a selected degree of rotationabout the third longitudinal axis of the substantially cylindricalsubstrate.

In some embodiments of the process, the first cutting head and thesecond cutting head may be separated by 180 degrees of rotation aboutthe third longitudinal axis of the substantially cylindrical substrate.

In some embodiments, a fastener may be formed by a process including:placing a first cutting head of a first mill tool at a first positionalong a shaft of the fastener having a proximal end and a distal end,placing a second cutting head of a second mill tool at a second positionalong the shaft, rotating the first cutting head about a firstlongitudinal axis of the first mill tool, rotating the second cuttinghead about a second longitudinal axis of the second mill tool, rotatingthe shaft about a third longitudinal axis of the shaft, and translatingthe first and second cutting heads along at least a part of a length ofthe shaft to form a first concave undercut surface oriented toward theproximal end, a first convex undercut surface oriented toward the distalend, a second concave undercut surface oriented toward the distal end,and a second convex undercut surface oriented toward the proximal end.

In some embodiments of the fastener formed by the process, translatingthe first and second cutting heads along at least part of the length ofthe shaft may form a first helical thread and a second helical thread.The first helical thread may include the first concave undercut surfaceand the first convex undercut surface. The second helical thread mayinclude the second concave undercut surface and the second convexundercut surface.

Some embodiments of the fastener formed by the process may also includeplacing the first cutting head adjacent the second cutting head along aside of the shaft with the first longitudinal axis of the first milltool substantially parallel to the second longitudinal axis of thesecond mill tool.

In some embodiments of the fastener formed by the process, the firstcutting head may include at least one convex cutting surface and thesecond cutting head may include at least one concave cutting surface.

In some embodiments of the fastener formed by the process, the at leastone convex cutting surface may include a first facet and a second facet,and the at least one concave cutting surface may include a third facetand a fourth facet. The first facet and the second facet may be angledwith respect to each other by a first angle that may be greater than 180degrees to form the at least one convex cutting surface, and the thirdfacet and the fourth facet may be angled with respect to each other by asecond angle that may be less than 180 degrees to form the at least oneconcave cutting surface.

In some embodiments of the fastener formed by the process, the processmay also include placing the first cutting head on a first side of theshaft and placing the second cutting head on a second side of the shaft.The first cutting head and the second cutting head may be separated by aselected degree of rotation about the third longitudinal axis of theshaft.

In some embodiments, an implantable bone anchor may be formed by aprocess including: placing a first cutting head of a first mill tool ata first position along a shaft of the implantable bone anchor having aproximal end and a distal end, placing a second cutting head of a secondmill tool at a second position along the shaft, rotating the firstcutting head about a first longitudinal axis of the first mill tool,rotating the second cutting head about a second longitudinal axis of thesecond mill tool, rotating the shaft about a third longitudinal axis ofthe shaft, and translating the first and second cutting heads along atleast a part of a length of the shaft to form a firstproximally-oriented surface facing toward a proximal end of the shaft, afirst distally-oriented surface facing toward a distal end of the shaft,a second proximally-oriented surface facing toward the proximal end ofthe shaft, and a second distally-oriented surface facing toward thedistal end of the shaft.

In some embodiments of the implantable bone anchor formed by theprocess, translating the first and second cutting heads along at least apart of a length of the shaft may form a first helical thread and asecond helical thread. The first helical thread may include the firstproximally-oriented surface facing toward a proximal end of the shaftand the first distally-oriented surface facing toward a distal end ofthe shaft. The second helical thread may include the secondproximally-oriented surface facing toward the proximal end of the shaftand the second distally-oriented surface facing toward the distal end ofthe shaft. In some embodiments, the first proximally-oriented surfaceand the first distally-oriented surface may not have mirror symmetryrelative to each other across any plane perpendicular to the thirdlongitudinal axis of the shaft. In some embodiments, the firstproximally-oriented surface and the second distally-oriented surface mayhave mirror symmetry relative to each other across a first planeperpendicular to the third longitudinal axis of the shaft.

In some embodiments of the implantable bone anchor formed by theprocess, the process may also include placing the first cutting headadjacent the second cutting head along a side of the shaft with thefirst longitudinal axis of the first mill tool substantially parallel tothe second longitudinal axis of the second mill tool.

In some embodiments of the implantable bone anchor formed by theprocess, the first cutting head may include at least one convex cuttingsurface and the second cutting head may include at least one concavecutting surface.

In some embodiments of the implantable bone anchor formed by theprocess, the at least one convex cutting surface may include a firstfacet and a second facet, and the at least one concave cutting surfacemay include a third facet and a fourth facet. The first facet and thesecond facet may be angled with respect to each other by a first anglethat may be greater than 180 degrees to form the at least one convexcutting surface, and the third facet and the fourth facet may be angledwith respect to each other by a second angle that may be less than 180degrees to form the at least one concave cutting surface.

In some embodiments of the implantable bone anchor formed by theprocess, the process may also include placing the first cutting head ona first side of the shaft and placing the second cutting head on asecond side of the shaft. The first cutting head and the second cuttinghead may be separated by a selected degree of rotation about the thirdlongitudinal axis of the shaft.

These and other features and advantages of the present disclosure willbecome more fully apparent from the following description and appendedclaims, or may be learned by the practice of the instruments, systems,and methods set forth hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present disclosure will become more fullyapparent from the following description taken in conjunction with theaccompanying drawings. Understanding that these drawings depict onlyexemplary embodiments and are, therefore, not to be considered limitingof the scope of the present disclosure, the exemplary embodiments of thepresent disclosure will be described with additional specificity anddetail through use of the accompanying drawings in which:

FIG. 1A illustrates a front perspective view of a fastener, according toan embodiment of the present disclosure; FIG. 1B illustrates a rearperspective view of the fastener of FIG. 1A;

FIG. 1C illustrates a side view of the fastener of FIG. 1A; FIG. 1Dillustrates a cross-sectional side view of the fastener of FIG. 1A takenalong the line A-A shown in FIG. 1C;

FIG. 2A illustrates a front perspective view of a fastener, according toanother embodiment of the present disclosure; FIG. 2B illustrates a rearperspective view of the fastener of FIG. 2A; FIG. 2C illustrates a sideview of the fastener of FIG. 2A; FIG. 2D illustrates a cross-sectionalside view of the fastener of FIG. 2A taken along the line B-B shown inFIG. 2C;

FIG. 3A illustrates a front perspective view of a fastener, according toanother embodiment of the present disclosure; FIG. 3B illustrates a rearperspective view of the fastener of FIG. 3A; FIG. 3C illustrates a sideview of the fastener of FIG. 3A; FIG. 3D illustrates a cross-sectionalside view of the fastener of FIG. 3A taken along the line C-C shown inFIG. 3C;

FIG. 4A illustrates a front perspective view of a mill tool, accordingto one embodiment of the present disclosure; FIG. 4B illustrates a rearperspective view of the mill tool of FIG. 4A;

FIG. 4C illustrates a side view of the mill tool of FIG. 4A; FIG. 4Dillustrates a front view of the mill tool of FIG. 4A;

FIG. 5A illustrates a front perspective view of a mill tool, accordingto another embodiment of the present disclosure; FIG. 5B illustrates arear perspective view of the mill tool of FIG. 5A; FIG. 5C illustrates aside view of the mill tool of FIG. 5A; FIG. 5D illustrates a front viewof the mill tool of FIG. 5A;

FIG. 6 illustrates a perspective view of the mill tool of FIG. 5Aperforming a milling operation on the fastener of FIG. 1A;

FIG. 7 illustrates a side view of the mill tool of FIG. 5A and the milltool of FIG. 4A performing milling operations on the fastener of FIG.1A;

FIG. 8 illustrates a cross-sectional side view of the mill tool of FIG.5A and the mill tool of FIG. 4A performing milling operations on thefastener of FIG. 1A;

FIG. 9 illustrates a flow diagram of a process for forming threading ona fastener, according to one embodiment of the present disclosure; and

FIG. 10 illustrates a partial cross-sectional side view of a fastenercomprising crescent-shaped threading.

It is to be understood that the drawings are for purposes ofillustrating the concepts of the present disclosure and may not be drawnto scale. Furthermore, the drawings illustrate exemplary embodiments anddo not represent limitations to the scope of the present disclosure.

DETAILED DESCRIPTION

Exemplary embodiments of the present disclosure will be best understoodby reference to the drawings, wherein like parts are designated by likenumerals throughout. It will be readily understood that the componentsof the present disclosure, as generally described and illustrated in thedrawings, could be arranged, and designed in a wide variety of differentconfigurations. Thus, the following more detailed description of theembodiments of the implants, systems, and methods, as represented in thedrawings, is not intended to limit the scope of the present disclosure,but is merely representative of exemplary embodiments of the presentdisclosure.

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any embodiment described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments. While the various aspects of theembodiments are presented in the drawings, the drawings are notnecessarily drawn to scale unless specifically indicated.

The following disclosure presents various fasteners for utilization inbone and other tissues as implantable devices (e.g., orthopedicimplants, spine implants, sports medicine implants, dental implants,trauma implants, reconstruction implants, extremity implants,craniomaxillofacial (CMF) implants, veterinary implants, etc.) for thepurpose of streamlining the present disclosure. However, it will beunderstood that the various fasteners and helical threading conceptspresented herein can be utilized in any medium beyond bones/tissuesand/or for any application beyond surgical procedures.

Example applications/procedures that may utilize any of the fastenersdescribed or contemplated herein, in any configuration and with any ofthe features described herein, may include, but are not limited to:trauma procedures, spine procedures (e.g., SI fusion, facet fixation,etc.), reconstruction procedures, sports related procedures,ACL/tenodesis procedures, extremity procedures, dental procedures, CMFprocedures, veterinary procedures, fracture fixation plate procedures(e.g., distal femur plates, proximal humerus plates, tibial plates,etc.), supplemental Fixation for IBD procedures, intramedullary canalfixation procedures, nail fixation procedures, limb salvage andtransfemoral procedures, amputee connection procedures, total shoulderfixation, reverse glenoid fixation, small bone fixation (e.g.,podiatric, hand/wrist, etc.), joint fusions, single-tooth implantfixation, jaw/facial reconstruction, dentures fixation, veterinarytrauma, species specific procedures (e.g., equine, canine, rabbit,etc.), TPLO, shear fixation, osteotomies, fusions, procedures involvingosteoporotic or compromised bone, etc.

Moreover, fastener types that may utilize any of the thread designs,morphology, and/or features described herein may include, but are notlimited to: cortical fasteners, soft tissue fasteners, long fasteners,cannulated fasteners, plate fasteners, locking/non-locking fasteners,dynamic hip fasteners, acetabular cup fasteners, Schanz pins, half pins,pedicle fasteners, cervical fasteners, threaded stems, threadedintramedullary canal stems, joint stems, revision fasteners, compressionfasteners (e.g., headless/headed compression fasteners, hip compressionfasteners, etc.), ACL fasteners, tenodesis fasteners, bone-tendon-bonegraft fasteners, suture anchors, dental fasteners, mandibular tentingfasteners, veterinary fasteners, etc.

FIGS. 1A-D illustrate various views of a fastener 100, implantable boneanchor, or bone screw, according to one embodiment of the presentdisclosure. Specifically, FIG. 1A is a front perspective view of thefastener 100, FIG. 1B is a rear perspective view of the fastener 100,FIG. 1C is a side view of the fastener 100, and FIG. 1D is across-sectional side view of the fastener 100 taken along the line A-Ain FIG. 1C.

In general, the fastener 100 may include a shaft 105 having a proximalend 101, a distal end 102, and a longitudinal axis 103. The fastener 100may also include a head 104 located at the proximal end 101 of the shaft105, a torque connection interface 106 formed in/on the head 104 (ineither a male/female configuration), and a self-tapping feature 107formed in the distal end 102 of the shaft 105.

In some embodiments, the fastener 100 may include a first helical thread110 disposed about the shaft 105, and a second helical thread 120disposed about the shaft 105 adjacent the first helical thread 110.

In some embodiments, the fastener 100 may include a “dual start” or“dual lead” thread configuration comprising the first helical thread 110and the second helical thread 120.

In some embodiments, a depth of the first helical thread 110 and/or thesecond helical thread 120 with respect to the shaft 105 may define amajor diameter vs. a minor diameter of the shaft 105 alone.

In some embodiments, a major diameter and/or a minor diameter of thefastener 100 may be constant or substantially constant along the entirelength of the fastener, or along a majority of the length of thefastener. In these embodiments, a constant minor diameter may help avoidblowout of narrow/delicate bones (e.g., a pedicle) when inserting afastener into a bone. In some embodiments, a pilot hole may first bedrilled into a narrow/delicate bone and then a fastener having a similarminor diameter in comparison to the diameter of the pilot hole may bechosen to avoid blowout when inserting the fastener into the bone.

In some embodiments, a depth of the first helical thread 110 and/or thesecond helical thread 120 with respect to the shaft 105 may vary along alength of the shaft 105 to define one or more major diameters of thefastener 100 and/or one or more regions along the fastener 100 maycomprise a one or more continuously variable major diameters.

In some embodiments, a thickness of the shaft 105 may vary along alength of the shaft 105 to define one or more minor diameters of thefastener 100, and/or one or more regions along the fastener 100 maycomprise one or more continuously variable minor diameters. In someembodiments, a thickness/height/width/length/pitch/shape of the firsthelical thread 110 and/or the second helical thread 120 (or anyadditional helical thread) may vary along a length of the shaft 105. Forexample, a thickness/height/width/length/pitch/shape of the firsthelical thread 110 and/or the second helical thread 120 may be greatertowards the tip of the fastener and thinner towards the head of thefastener (or vice versa) in either a discrete or continuously variablefashion, etc.

In some embodiments, the major and/or minor diameters may increasetoward a proximal end or head of a fastener in order to increase bonecompaction as the fastener is terminally inserted into the bone/tissue.

In some embodiments, a pitch of the first helical thread 110 and/or thesecond helical thread 120 may vary along a length of the fastener 100.

In some embodiments, the fastener 100 may include a plurality of helicalthreads disposed about the shaft 105. However, it will also beunderstood that any of the fasteners disclosed or contemplated hereinmay include a single helical thread disposed about the shaft of thefastener. Moreover, the fastener 100 may comprise a nested plurality ofhelical threads having different lengths (not shown). As onenon-limiting example, the fastener 100 may include a first helicalthread 110 that is longer than a second helical thread 120, such thatthe fastener 100 comprises dual threading along a first portion of theshaft 105 and single threading along a second portion of the shaft 105.

In some embodiments, the plurality of helical threads may include threehelical threads (not shown) comprising a “triple start” or “triple lead”thread configuration (not shown).

In some embodiments, the plurality of helical threads may include fourhelical threads (not shown) comprising a “quadruple start” or “quadruplelead” thread configuration (not shown).

In some embodiments, the plurality of helical threads may include morethan four helical threads (not shown).

In some embodiments, the fastener 100 may include first threading withany of the shapes disclosed herein oriented toward one of the proximalend and the distal end of the fastener 100 with the first threadinglocated proximate the distal end of the fastener 100, as well as secondthreading with any of the shapes disclosed herein oriented toward theother one of the proximal end and the distal end of the fastener 100with the second threading located proximate the head of the fastener 100(not shown).

In some embodiments, the fastener 100 may include multiple threading(e.g., dual helical threading, etc.) with any of the shapes disclosedherein located proximate one of the proximal end and the distal end ofthe fastener 100, as well as single threading with any of the shapesdisclosed herein with the second threading located proximate the otherof the proximal end and the distal end of the fastener 100 (not shown).

In some embodiments, the first helical thread 110 may include aplurality of first concave undercut surfaces 131 and a plurality offirst convex undercut surfaces 141.

In some embodiments, the second helical thread 120 may include aplurality of second concave undercut surfaces 132 and a plurality ofsecond convex undercut surfaces 142.

In some embodiments, when the fastener 100 is viewed in section along aplane that intersects the longitudinal axis 103 of the shaft 105 (asshown in FIG. 1D), the plurality of first concave undercut surfaces 131and the plurality of second convex undercut surfaces 142 may be orientedtoward (i.e., point toward) the proximal end 101 of the shaft 105.

In some embodiments, the plurality of first convex undercut surfaces 141and the plurality of second concave undercut surfaces 132 may beoriented toward (i.e., point toward) the distal end 102 of the shaft105.

In some embodiments, at least one of the plurality of first concaveundercut surfaces 131, the plurality of first convex undercut surfaces141, the plurality of second concave undercut surfaces 132, and theplurality of second convex undercut surfaces 142 may comprise at leastone substantially flat surface.

In some embodiments, when the fastener 100 is viewed in section along aplane intersecting the longitudinal axis 103 of the shaft 105, the firsthelical thread 110 may comprise a plurality of first bent shapes(comprising at least one surface that is angled relative to thelongitudinal axis 103 of the shaft 105 and/or at least one undercutsurface) with a plurality of first intermediate portions 151 that areoriented toward (i.e., point toward) the distal end 102 of the shaft105. This may be referred to as “standard” threading, having a“standard” orientation.

In some embodiments, when the fastener 100 is viewed in section along aplane intersecting the longitudinal axis 103 of the shaft 105, thesecond helical thread 120 may comprise a plurality of second bent shapes(comprising at least one surface that is angled relative to thelongitudinal axis 103 of the shaft 105 and/or at least one undercutsurface) with a plurality of second intermediate portions 152 that areoriented toward (i.e., point toward) the proximal end 101 of the shaft105. This may be referred to as “inverted” threading, having an“inverted” orientation.

In some embodiments, one or more helical threads may morph/transitionbetween a standard orientation and an inverted orientation along a shaftof a fastener.

In some embodiments, at least one of the plurality of first concaveundercut surfaces 131, the plurality of first convex undercut surfaces141, the plurality of second concave undercut surfaces 132, and theplurality of second convex undercut surfaces 142 may comprise at leastone curved surface.

As shown in FIG. 1D, the proximally-oriented and distally-orientedsurfaces of the first helical thread 110 (i.e., the first concaveundercut surfaces 131 and the first convex undercut surfaces 141 in thefastener 100 of FIG. 1D) may not have mirror symmetry relative to eachother about any plane perpendicular to the longitudinal axis 103 of thefastener 100. Rather, the first concave undercut surfaces 131 and thefirst convex undercut surfaces 141 may be generally parallel to eachother. The same may be true for the second helical thread 120, in whichthe second concave undercut surfaces 132 and the second convex undercutsurfaces 142 do not have mirror symmetry relative to each other, but maybe generally parallel to each other.

Conversely, as also shown in FIG. 1D, the proximally-oriented surfacesof the first helical thread 110 may have mirror symmetry relative to thedistally-oriented surfaces of the second helical thread 120.Specifically, the first concave undercut surfaces 131 may have mirrorsymmetry relative to the second convex undercut surfaces 142 about aplane 170 that bisects the space between them, and lies perpendicular tothe longitudinal axis 103.

Similarly, the distally-oriented surfaces of the first helical thread110 may have mirror symmetry relative to the proximally-orientedsurfaces of the second helical thread 120. Specifically, the secondconcave undercut surfaces 132 may have mirror symmetry relative to thefirst convex undercut surfaces 141 about a plane 172 that bisects thespace between them, and lies perpendicular to the longitudinal axis 103.

This mirror symmetry may be present along most of the length of thefirst helical thread 110 and the second helical thread 120, withsymmetry across different planes arranged between adjacent turns of thefirst helical thread 110 and the second helical thread 120 along thelength of the longitudinal axis 103. Such mirror symmetry may help moreeffectively capture bone between the first helical thread 110 and thesecond helical thread 120, and may also facilitate manufacture of thefastener 100, as will be described in more detail below.

In some embodiments, when the fastener 100 is viewed in section along aplane intersecting the longitudinal axis 103 of the shaft 105, the firsthelical thread 110 may include at least one partial crescent shape thatis oriented toward (i.e., points toward) the distal end 102 of the shaft105 and/or the proximal end 101 of the shaft 105. FIG. 10 illustrates apartial cross-sectional view of a fastener 700 comprising crescentshapes, as one non-limiting example of such an embodiment.

In some embodiments (not shown), when the fastener 100 is viewed insection along a plane intersecting the longitudinal axis 103 of theshaft 105, the first helical thread 110 may include at least one partialcrescent shape that is oriented toward (i.e., points toward) the distalend 102 of the shaft 105, and the second helical thread 120 may includeat least one partial crescent shape that is oriented toward (i.e.,points toward) the proximal end 101 of the shaft 105.

In some embodiments (not shown), the first helical thread 110 mayinclude a first plurality of partial crescent shapes that are orientedtoward (i.e., point toward) the distal end 102 of the shaft 105, and thesecond helical thread 120 may include a second plurality of partialcrescent shapes that are oriented toward (i.e., point toward) theproximal end 101 of the shaft 105.

In some embodiments (not shown), the first plurality of partial crescentshapes and the second plurality of partial crescent shapes may bearranged in alternating succession along the shaft 105 of the fastener100.

In some embodiments, a fastener may have only standard threads or onlyinverted threads. The type of threads that are desired may depend on thetype and/or magnitude of loads to be applied to the fastener. Forexample, a screw loaded axially away from the bone in which it isimplanted may advantageously have a standard thread, while a screwloaded axially toward the bone in which it is implanted mayadvantageously have an inverted thread. A screw that may experiencemulti-axial loading and/or off-loading conditions may advantageouslyinclude at least one standard thread and at least one inverted thread inorder to increase bone fixation and load sharing between a bone/fastenerinterface during multi-axial and off-loading conditions to reduce highbone strain and distribute multi-axial forces applied to the bone in aload-sharing, rather than load-bearing, configuration. Shear loadsand/or bending moments may also be optimally resisted with any chosencombination of threading, threading morphology, and/or threadingvariations contemplated herein to optimally resist shear loads, bendingmoments, multi-axial loading, off-loading conditions, etc.

In some embodiments, fasteners with standard threads may be used inconjunction with fasteners with inverted threads in order to accommodatedifferent loading patterns.

In some embodiments, a single fastener may have both standard andinverted threads, like the fastener 100. Such a combination of threadsmay help the fastener 100 remain in place with unknown and/or varyingloading patterns.

In some embodiments, the geometry of the threading of a fastener (withstandard and/or inverted threads) may be varied to suit the fastener fora particular loading scheme. For example, the number of threads, thenumber of thread starts, the pitch of the threading, the lead(s) of thethreading, the shape(s) of the threading, any dimension(s) associatedwith the threading (e.g., any length(s)/width(s)/height(s) associatedwith the threading), the major diameter(s), the minor diameter(s), anyangulation/angles associated with any surfaces of the threading, the“handedness” of the threading (e.g., right-handed vs. left-handed),etc., may be varied accordingly to suit any specific medium ofinstallation, loading pattern, application, procedure, etc., that may beinvolved.

In some embodiments, the material(s) of any portion of a fastenerdescribed herein may include, but are not limited to: metals (e.g.,titanium, cobalt, stainless steel, etc.), metal alloys, plastics,polymers, PEEK, UHMWPE, composites, additive particles, texturedsurfaces, biologics, biomaterials, bone, etc.

In some embodiments, any of the fasteners described herein may includeadditional features such as: self-tapping features, locking features(e.g., locking threading formed on a portion of the fastener, such asthreading located on or near a head of the fastener), cannulation, anystyle of fastener head (or no fastener head at all), any style of torqueconnection interface (or no torque connection interface at all), etc.

In some embodiments, a tap (not shown) may be utilized to pre-formthreading herein in a bone according to any threading shape that isdisclosed herein. In this manner, taps with any suitable shape may beutilized in conjunction with any fastener described or contemplatedherein to match or substantially match the threading geometry of a givenfastener.

In some embodiments, a minor diameter of the fastener may be selected tomatch, or substantially match, a diameter of a pilot hole that is formedin a bone to avoid bone blowout when the fastener is inserted into thepilot hole.

Additionally or alternatively, the type of threads and/or threadgeometry may be varied based on the type of bone in which the fasteneris to be anchored. For example, fasteners anchored in osteoporotic bonemay fare better with standard or inverted threads, or when the pitch,major diameter, and/or minor diameter are increased or decreased, orwhen the angulation of thread surfaces is adjusted, etc.

In some embodiments, a surgical kit may include multiple fasteners withany of the different thread options described or contemplated herein.The surgeon may select the appropriate fastener(s) from the kit based onthe particular loads to be applied and/or the quality of bone in whichthe fastener(s) are to be anchored.

Continuing with FIG. 1D, in some embodiments the first helical thread110 may include a plurality of first undercut surfaces 111, a pluralityof second undercut surfaces 112, a plurality of third undercut surfaces113, and a plurality of fourth open surfaces 114.

In some embodiments, the second helical thread 120 may include aplurality of fifth undercut surfaces 125, a plurality of sixth undercutsurfaces 126, a plurality of seventh undercut surfaces 127, and aplurality of eighth open surfaces 128.

In some embodiments one or more of the plurality of first undercutsurfaces 111, the plurality of second undercut surfaces 112, theplurality of third undercut surfaces 113, the plurality of fourth opensurfaces 114, the plurality of fifth undercut surfaces 125, theplurality of sixth undercut surfaces 126, the plurality of seventhundercut surfaces 127, and the plurality of eighth open surfaces 128 maycomprise at least one flat or substantially flat surface.

In some embodiments, the plurality of first undercut surfaces 111, theplurality of third undercut surfaces 113, the plurality of sixthundercut surfaces 126, and the plurality of eighth open surfaces 128 maybe angled towards the distal end 102 of the shaft 105.

In some embodiments, the plurality of second undercut surfaces 112, theplurality of fourth open surfaces 114, the plurality of fifth undercutsurfaces 125, and the plurality of seventh undercut surfaces 127 may beangled towards the proximal end 101 of the shaft 105.

In some embodiments, when the fastener 100 is viewed in section along aplane that intersects the longitudinal axis 103 of the shaft 105 (asshown in FIG. 1D), the first helical thread 110 may include at least onechevron shape that is oriented toward (i.e., points toward) the distalend 102 of the shaft 105. Likewise, the second helical thread 120 mayalso include at least one chevron shape that is oriented toward (i.e.,points toward) the proximal end 101 of the shaft 105.

In some embodiments, when the fastener 100 is viewed in section along aplane that intersects the longitudinal axis 103 of the shaft 105 (asshown in FIG. 1D), the first helical thread 110 may include a firstplurality of chevron shapes that are oriented toward (i.e., pointtoward) the distal end 102 of the shaft 105. Likewise, the secondhelical thread 120 may include a second plurality of chevron shapes thatare oriented toward (i.e., point toward) the proximal end 101 of theshaft 105.

In some embodiments, the first plurality of chevron shapes and thesecond plurality of chevron shapes may be arranged in alternatingsuccession along the shaft 105 of the fastener 100, (e.g., see FIG. 1D).

In some embodiments, a plurality of first interlocking spaces 161 and aplurality of second interlocking spaces 162 may be formed between thefirst helical thread 110 and the second helical thread 120 along theshaft 105 of the fastener 100.

In some embodiments, the plurality of first interlocking spaces 161 maybe formed intermediate the first concave undercut surfaces 131 and thesecond concave undercut surfaces 132.

In some embodiments, the plurality of second interlocking spaces 162 maybe formed intermediate the first convex undercut surfaces 141 and thesecond convex undercut surfaces 142.

In some embodiments, the plurality of first interlocking spaces 161 maybe larger in size than the plurality of second interlocking spaces.

In some embodiments, the plurality of first interlocking spaces 161 andthe plurality of second interlocking spaces 162 may be shaped and/orconfigured to interlock with bone/other tissues received therein toincrease fixation of the fastener 100 within the bone/other tissues andprovide additional resistance against multi-axial forces that may beapplied to the fastener 100 and/or the bone/other tissues.

In some embodiments, the plurality of second undercut surfaces 112 andthe plurality of sixth undercut surfaces 126 may be angled toward eachother to trap bone/other tissues within the plurality of firstinterlocking spaces 161 in order to increase fixation and resistanceagainst multi-axial forces.

In some embodiments, the plurality of third undercut surfaces 113 andthe plurality of seventh undercut surfaces 127 may be angled toward eachother to trap bone/other tissues within the plurality of secondinterlocking spaces 162 in order to increase fixation and resistanceagainst multi-axial forces.

In some embodiments, the plurality of first undercut surfaces 111 andthe plurality of fifth undercut surfaces 125 may each form an angle αwith respect to the longitudinal axis 103 of the shaft 105, as shown inFIG. 1D.

In some embodiments, the angle α may be greater than 90 degrees.

In some embodiments, the plurality of second undercut surfaces 112 andthe plurality of sixth undercut surfaces 126 may each form an angle βwith respect to the longitudinal axis 103 of the shaft 105.

In some embodiments, the angle β may be less than 90 degrees.

In some embodiments, the plurality of third undercut surfaces 113 andthe plurality of seventh undercut surfaces 127 may each form an angle θwith respect to the longitudinal axis 103 of the shaft 105.

In some embodiments, the angle θ may be approximately 90 degrees.

In some embodiments, the angle θ may be greater than 90 degrees.

FIGS. 2A-D illustrate various views of a polyaxial screw or fastener200, according to another embodiment of the present disclosure.Specifically, FIG. 2A is a front perspective view of the fastener 200,FIG. 2B is a rear perspective view of the fastener 200, FIG. 2C is aside view of the fastener 200, and FIG. 2D is a cross-sectional sideview of the fastener 200 taken along the line B-B in FIG. 2C. Thefastener 200 may include a shaft 205 having a proximal end 201, a distalend 202, and a longitudinal axis 203. The fastener 200 may also includea polyaxial head 204 located at the proximal end 201 of the shaft 205, atorque connection interface 206 formed in/on the polyaxial head 204, anda self-tapping feature 207 formed in the distal end 202 of the shaft205. In some embodiments, the fastener 200 may include a first helicalthread 210 disposed about the shaft 205, and a second helical thread 220disposed about the shaft 205 adjacent the first helical thread 210. Inthese embodiments, the fastener 200 may comprise a “dual start” or “duallead” thread configuration. However, it will also be understood that thefastener 200 may include any thread configuration, feature, ormorphology described or contemplated herein to achieve optimal fixationwithin a given bone/tissue.

FIGS. 3A-D illustrate various views of a headless screw or fastener 300,according to another embodiment of the present disclosure. Specifically,FIG. 3A is a front perspective view of the fastener 300, FIG. 3B is arear perspective view of the fastener 300, FIG. 3C is a side view of thefastener 300, and FIG. 3D is a cross-sectional side view of the fastener300 taken along the line C-C in FIG. 3C. The fastener 300 may include ashaft 305 having a proximal end 301, a distal end 302, and alongitudinal axis 303. The fastener 300 may also include a torqueconnection interface 306 formed in the proximal end 301 of the shaft 305and a self-tapping feature 307 formed in the distal end 302 of the shaft305. In some embodiments, the fastener 300 may include a first helicalthread 310 disposed about the shaft 305, and a second helical thread 320disposed about the shaft 305 adjacent the first helical thread 310. Inthese embodiments, the fastener 300 may comprise a “dual start” or “duallead” thread design with alternating standard and inverted threads.However, it will also be understood that the fastener 300 may includeany thread configuration, feature, or morphology described orcontemplated herein to achieve optimal fixation within a givenbone/tissue.

FIGS. 4A-D illustrate various views of a first mill tool 400, accordingto one embodiment of the present disclosure. Specifically, FIG. 4A is afront perspective view of the first mill tool 400, FIG. 4B is a rearperspective view of the first mill tool 400, FIG. 4C is a side view ofthe first mill tool 400, and FIG. 4D is a front view of the first milltool of FIG. 400. The first mill tool 400 may include a shaft 405 havinga proximal end 401, a distal end 402, and a longitudinal axis 403.

In some embodiments, the first mill tool 400 may include a first cuttinghead 410 comprising one or more first cutting blades 420 disposed at thedistal end 402 of the shaft 405.

In some embodiments, the one or more first cutting blades 420 maycomprise at least one convex cutting surface 430.

In some embodiments, the at least one convex cutting surface 430 maycomprise a first facet 431 and a second facet 432 (e.g., see FIGS. 4Aand 4C, as one non-limiting example).

In some embodiments, the first facet 431 and/or the second facet 432 maycomprise one or more flat surfaces.

In some embodiments, the first facet 431 and/or the second facet 432 maycomprise one or more curved surfaces.

In some embodiments, the first facet 431 and the second facet 432 may beangled with respect to each other to form the at least one convexcutting surface 430.

In some embodiments, the first facet 431 and the second facet 432 may beangled with respect to each other by a first angle 441 that may begreater than 180 degrees to form the at least one convex cutting surface430.

FIGS. 5A-D illustrate various views of a second mill tool 500, accordingto another embodiment of the present disclosure. Specifically, FIG. 5Ais a front perspective view of the second mill tool 500, FIG. 5B is arear perspective view of the second mill tool 500, FIG. 5C is a sideview of the second mill tool 500, and FIG. 5D is a front view of thesecond mill tool of FIG. 500. The second mill tool 500 may include ashaft 505 having a proximal end 501, a distal end 502, and alongitudinal axis 503.

In some embodiments, the second mill tool 500 may include a secondcutting head 510 comprising one or more second cutting blades 520disposed at the distal end 502 of the shaft 505.

In some embodiments, the one or more second cutting blades 520 maycomprise at least one concave cutting surface 530.

In some embodiments, the at least one concave cutting surface 530 maycomprise a third facet 533 and a fourth facet 534 (e.g., see FIGS. 5Aand 5C, as one non-limiting example).

In some embodiments, the third facet 533 and/or the fourth facet 534 maycomprise one or more flat surfaces.

In some embodiments, the third facet 533 and/or the fourth facet 534 maycomprise one or more curved surfaces.

In some embodiments, the third facet 533 and the fourth facet 534 may beangled with respect to each other to form the at least one concavecutting surface 530.

In some embodiments, the third facet 533 and the fourth facet 534 may beangled with respect to each other by a second angle 542 that may be lessthan 180 degrees to form the at least one concave cutting surface 530.

FIGS. 6-8 illustrate various views of the first mill tool 400 and thesecond mill tool 500 performing milling operations on the fastener 100of FIG. 1A to form the first helical thread 110 and the second helicalthread 120. Specifically, FIG. 6 is a perspective view of the secondmill tool 500 performing a milling operation on the fastener 100, FIG. 7is a side view of both the first mill tool 400 and the second mill tool500 performing milling operations on the fastener 100 simultaneously,and FIG. 8 is a cross-sectional side view of both the first mill tool400 and the second mill tool 500 performing milling operations on thefastener 100 simultaneously. FIG. 8 illustrates how the first mill tool400 and the second mill tool 500 may be utilized to form the pluralityof first interlocking spaces 161 and the plurality of secondinterlocking spaces 162 between the first helical thread 110 and thesecond helical thread 120. Various milling techniques that may beutilized to form the helical threading described herein will now bedescribed in more detail with reference to FIG. 9.

FIG. 9 illustrates a flow diagram of a process or method 600 of formingthreading on a shaft or a substantially cylindrical substrate (notshown) to create a threaded fastener or implantable bone anchor,according to some embodiments of the present disclosure.

In some embodiments, the method 600 may begin with a step 610 in which afirst cutting head of a first mill tool may be placed at a firstposition along the substantially cylindrical substrate.

In some embodiments of the method 600, a second cutting head of a secondmill tool may, alternatively, or in addition thereto, be placed at asecond position along the substantially cylindrical substrate in a step620 at the same time (or at a different time) as the first cutting headis placed at the first position. However, it will also be understoodthat, in some embodiments, any number of cutting heads/mill tools may beutilized to form any number of helical threads (as previously discussed)about the substantially cylindrical substrate, in either a successive ora simultaneous manner.

In some embodiments, the first cutting head may be placed adjacent thesecond cutting head along a side of the substantially cylindricalsubstrate.

In some embodiments, the first longitudinal axis of the first mill toolmay be placed substantially parallel to the second longitudinal axis ofthe second mill tool.

In some embodiments, the first cutting head may be spaced apart from thesecond cutting head along the substantially cylindrical substrate.

In some embodiments, the first cutting head may be placed opposite thesecond cutting head along opposing sides of the substantiallycylindrical substrate.

In some embodiments, the first cutting head may be placed on a firstside of the substantially cylindrical substrate and the second cuttinghead may be placed on a second side of the substantially cylindricalsubstrate.

In some embodiments, the first cutting head may be separated from thesecond cutting head by any selected degree of rotation about the thirdlongitudinal axis of the substantially cylindrical substrate.

In some embodiments, the first cutting head and the second cutting headmay be placed on opposing sides of the substantially cylindricalsubstrate and/or may be separated from each other by about 180 degreesof rotation with respect to the third longitudinal axis of thesubstantially cylindrical substrate.

Once the first mill tool and/or the second mill tool have been placedalong the substantially cylindrical substrate at their respectivepositions, the method 600 may proceed to one or more of a step 630, astep 640, and/or a step 650 in which the first cutting head may berotated about a first longitudinal axis of the first mill tool, thesecond cutting head may be rotated about a second longitudinal axis ofthe second mill tool, and the substantially cylindrical substrate may berotated about a third longitudinal axis of the substantially cylindricalsubstrate.

Once the first mill tool, the second mill tool, and the substantiallycylindrical substrate are all rotating about their respective axes, themethod 600 may proceed a step 660 in which the first and/or the secondcutting heads may be translated along a length of the substantiallycylindrical substrate. Alternatively, or in addition thereto, thesubstantially cylindrical substrate may be translated with respect tothe first and/or the second cutting heads.

As the first mill tool, the second mill tool, and/or the substantiallycylindrical substrate are translated with respect to each other duringrotation, the first helical thread may be formed in the substantiallycylindrical substrate in a step 670, and/or the second helical threadmay be formed in the substantially cylindrical substrate in a step 680by the cutting heads of the first and second mill tools.

Once a desired plurality of helical threading has been formed in thesubstantially cylindrical substrate, the method 600 may end.

Any procedures/methods disclosed herein comprise one or more steps oractions for performing the described method. The method steps and/oractions may be interchanged with one another. In other words, unless aspecific order of steps or actions is required for proper operation ofthe embodiment, the order and/or use of specific steps and/or actionsmay be modified.

Any of the fasteners described herein may be configured for removal andreplacement during a revision procedure by simply unscrewing andremoving the fastener from the bone/tissue in which the fastenerresides. Moreover, the fasteners described herein may advantageously beremoved from bone without removing any appreciable amount of bone duringthe removal process to preserve the bone. In this manner, implants maybe mechanically integrated with the bone, while not being cemented tothe bone or integrated via bony ingrowth, in order to provide an instantand removable connection between an implant and a bone. Accordingly,revision procedures utilizing the fasteners described herein can resultin less trauma to the bone and improved patient outcomes.

Reference throughout this specification to “an embodiment” or “theembodiment” means that a particular feature, structure, orcharacteristic described in connection with that embodiment is includedin at least one embodiment. Thus, the quoted phrases, or variationsthereof, as recited throughout this specification are not necessarilyall referring to the same embodiment.

Similarly, it should be appreciated that in the above description ofembodiments, various features are sometimes grouped together in a singleembodiment, Figure, or description thereof for the purpose ofstreamlining the present disclosure. This method of disclosure, however,is not to be interpreted as reflecting an intention that any embodimentrequires more features than those expressly recited in that embodiment.Rather, inventive aspects lie in a combination of fewer than allfeatures of any single foregoing disclosed embodiment.

Recitation of the term “first” with respect to a feature or element doesnot necessarily imply the existence of a second or additional suchfeature or element. Elements recited in means-plus-function format areintended to be construed in accordance with 35 U.S.C. § 112(f). It willbe apparent to those having skill in the art that changes may be made tothe details of the above-described embodiments without departing fromthe underlying principles set forth herein.

The phrases “connected to,” “coupled to” and “in communication with”refer to any form of interaction between two or more entities, includingmechanical, electrical, magnetic, electromagnetic, fluid, and thermalinteraction. Two components may be functionally coupled to each othereven though they are not in direct contact with each other. The term“coupled” can include components that are coupled to each other viaintegral formation, as well as components that are removably and/ornon-removably coupled with each other. The term “abutting” refers toitems that may be in direct physical contact with each other, althoughthe items may not necessarily be attached together. The phrase “fluidcommunication” refers to two or more features that are connected suchthat a fluid within one feature is able to pass into another feature.Moreover, as defined herein the term “substantially” means within +/−20%of a target value, measurement, or desired characteristic.

While specific embodiments and applications of the present disclosurehave been illustrated and described, it is to be understood that thescope of this disclosure is not limited to the precise configuration andcomponents disclosed herein. Various modifications, changes, andvariations which will be apparent to those skilled in the art may bemade in the arrangement, operation, and details of the devices, systems,and methods disclosed herein.

What is claimed is:
 1. A process for forming a fastener comprising:placing a first cutting head of a first mill tool at a first positionalong a substantially cylindrical substrate having a proximal end and adistal end; placing a second cutting head of a second mill tool at asecond position along the substantially cylindrical substrate; rotatingthe first cutting head about a first longitudinal axis of the first milltool; rotating the second cutting head about a second longitudinal axisof the second mill tool; rotating the substantially cylindricalsubstrate about a third longitudinal axis of the substantiallycylindrical substrate; translating the first and second cutting headsalong at least part of a length of the substantially cylindricalsubstrate to form: a first concave undercut surface oriented toward theproximal end; a first convex undercut surface oriented toward the distalend; a second concave undercut surface oriented toward the distal end;and a second convex undercut surface oriented toward the proximal end.2. The process of claim 1, wherein translating the first and secondcutting heads along at least part of the length of the substantiallycylindrical substrate forms: a first helical thread comprising: thefirst concave undercut surface; and the first convex undercut surface;and a second helical thread comprising: the second concave undercutsurface; and the second convex undercut surface.
 3. The process of claim1, comprising: placing the first cutting head adjacent the secondcutting head along a side of the substantially cylindrical substratewith the first longitudinal axis of the first mill tool substantiallyparallel to the second longitudinal axis of the second mill tool.
 4. Theprocess of claim 1, wherein: the first cutting head comprises at leastone convex cutting surface; and the second cutting head comprises atleast one concave cutting surface.
 5. The process of claim 4, wherein:the at least one convex cutting surface comprises: a first facet; and asecond facet; and the at least one concave cutting surface comprises: athird facet; and a fourth facet; wherein: the first facet and the secondfacet are angled with respect to each other by a first angle that isgreater than 180 degrees to form the at least one convex cuttingsurface; and the third facet and the fourth facet are angled withrespect to each other by a second angle that is less than 180 degrees toform the at least one concave cutting surface.
 6. The process of claim1, comprising: placing the first cutting head on a first side of thesubstantially cylindrical substrate; and placing the second cutting headon a second side of the substantially cylindrical substrate; wherein thefirst cutting head and the second cutting head are separated by aselected degree of rotation about the third longitudinal axis of thesubstantially cylindrical substrate.
 7. The process of claim 6,comprising placing the first cutting head opposite the second cuttinghead along opposing sides of the substantially cylindrical substrate. 8.The process of claim 7, wherein the first cutting head and the secondcutting head are separated by 180 degrees of rotation about the thirdlongitudinal axis of the substantially cylindrical substrate.
 9. Afastener formed by a process comprising: placing a first cutting head ofa first mill tool at a first position along a shaft of the fastenerhaving a proximal end and a distal end; placing a second cutting head ofa second mill tool at a second position along the shaft; rotating thefirst cutting head about a first longitudinal axis of the first milltool; rotating the second cutting head about a second longitudinal axisof the second mill tool; rotating the shaft about a third longitudinalaxis of the shaft; translating the first and second cutting heads alongat least a part of a length of the shaft to form: a first concaveundercut surface oriented toward the proximal end; a first convexundercut surface oriented toward the distal end; a second concaveundercut surface oriented toward the distal end; and a second convexundercut surface oriented toward the proximal end.
 10. The fastenerformed by the process of claim 9, wherein translating the first andsecond cutting heads along at least part of the length of the shaftforms: a first helical thread comprising: the first concave undercutsurface; and the first convex undercut surface; and a second helicalthread comprising: the second concave undercut surface; and the secondconvex undercut surface.
 11. The fastener formed by the process of claim9 further comprising: placing the first cutting head adjacent the secondcutting head along a side of the shaft with the first longitudinal axisof the first mill tool substantially parallel to the second longitudinalaxis of the second mill tool.
 12. The fastener formed by the process ofclaim 9, wherein: the first cutting head comprises at least one convexcutting surface; and the second cutting head comprises at least oneconcave cutting surface.
 13. The fastener formed by the process of claim12, wherein: the at least one convex cutting surface comprises: a firstfacet; and a second facet; and the at least one concave cutting surfacecomprises: a third facet; and a fourth facet; wherein: the first facetand the second facet are angled with respect to each other by a firstangle that is greater than 180 degrees to form the at least one convexcutting surface; and the third facet and the fourth facet are angledwith respect to each other by a second angle that is less than 180degrees to form the at least one concave cutting surface.
 14. Thefastener formed by the process of claim 9 further comprising: placingthe first cutting head on a first side of the shaft; and placing thesecond cutting head on a second side of the shaft; wherein the firstcutting head and the second cutting head are separated by a selecteddegree of rotation about the third longitudinal axis of the shaft. 15.An implantable bone anchor formed by a process comprising: placing afirst cutting head of a first mill tool at a first position along ashaft of the implantable bone anchor having a proximal end and a distalend; placing a second cutting head of a second mill tool at a secondposition along the shaft; rotating the first cutting head about a firstlongitudinal axis of the first mill tool; rotating the second cuttinghead about a second longitudinal axis of the second mill tool; rotatingthe shaft about a third longitudinal axis of the shaft; translating thefirst and second cutting heads along at least a part of a length of theshaft to form: a first proximally-oriented surface facing toward aproximal end of the shaft; a first distally-oriented surface facingtoward a distal end of the shaft; a second proximally-oriented surfacefacing toward the proximal end of the shaft; and a seconddistally-oriented surface facing toward the distal end of the shaft. 16.The implantable bone anchor formed by the process of claim 15, whereintranslating the first and second cutting heads along at least a part ofa length of the shaft forms: a first helical thread comprising: thefirst proximally-oriented surface facing toward a proximal end of theshaft; and the first distally-oriented surface facing toward a distalend of the shaft; and a second helical thread comprising: the secondproximally-oriented surface facing toward the proximal end of the shaft;and the second distally-oriented surface facing toward the distal end ofthe shaft; wherein: the first proximally-oriented surface and the firstdistally-oriented surface do not have mirror symmetry relative to eachother across any plane perpendicular to the third longitudinal axis ofthe shaft; and the first proximally-oriented surface and the seconddistally-oriented surface have mirror symmetry relative to each otheracross a first plane perpendicular to the third longitudinal axis of theshaft.
 17. The implantable bone anchor formed by the process of claim 15further comprising: placing the first cutting head adjacent the secondcutting head along a side of the shaft with the first longitudinal axisof the first mill tool substantially parallel to the second longitudinalaxis of the second mill tool.
 18. The implantable bone anchor formed bythe process of claim 15, wherein: the first cutting head comprises atleast one convex cutting surface; and the second cutting head comprisesat least one concave cutting surface.
 19. The implantable bone anchorformed by the process of claim 18, wherein: the at least one convexcutting surface comprises: a first facet; and a second facet; and the atleast one concave cutting surface comprises: a third facet; and a fourthfacet; wherein: the first facet and the second facet are angled withrespect to each other by a first angle that is greater than 180 degreesto form the at least one convex cutting surface; and the third facet andthe fourth facet are angled with respect to each other by a second anglethat is less than 180 degrees to form the at least one concave cuttingsurface.
 20. The implantable bone anchor formed by the process of claim15 further comprising: placing the first cutting head on a first side ofthe shaft; and placing the second cutting head on a second side of theshaft; wherein the first cutting head and the second cutting head areseparated by a selected degree of rotation about the third longitudinalaxis of the shaft.